1. Field of the Invention
This invention relates generally to the printing of moving webs, and is concerned in particular with improved paper printing and coating processes employing, in the most favorable embodiments, pigmented water based foamable ink or coating compositions.
2. Description of the Prior Art
The invention is especially useful in, but not limited to, rotogravure printing. Since its inception in the late 1800's, rotogravure printing has been carried out primarily with solvent based inks. The solvents are toxic and highly flammable, and consequently printing plant personnel are exposed continuously to potentially serious health, fire and explosion hazards. As a result, in spite of the implementation of elaborate and expensive protective measures such as for example solvent recovery and air pollution abatement systems, electrically grounded presses, shielded electric motors, etc., insurance costs have remained extremely high. Moreover, because solvents are extracted from petroleum, their costs have increased dramatically during the last decade, as has the cost of the energy consumed by the dryers required to dry the solvent inks after printing.
Some attempts have been made at avoiding these problems by employing water based inks. However, water based inks have a tendency to oversaturate the paper web. Moreover, water has a latent heat of vaporization five times that of hydrocarbon solvents, thus requiring five times as much energy to evaporate. For these reasons, the use of water based inks has not been widespread. Another attempted solution is disclosed in U.S. Pat. No. 2,971,458 (Kumins, et al), which suggests replacing solvent based inks with foamed water based inks applied directly to the web surface. To accomplish this direct foam application, Kumins stresses the importance of maintaining the size of the foam bubbles as nearly uniform as possible with the bubbles being considerably smaller than the finest engraved cells in the printing cylinder. While not specifically stated in Kumins, it would appear that these teachings are based on the theory that all of the engraved cells must be filled with or at least contain some foam bubbles in order to insure that the foam bubbles are applied to the web surface in the desired printing pattern.
It must be appreciated, however, that in rotogravure paper printing and coating, where for example the gravure cylinder has been electronically engraved by use of a Helio-Klischograph, cell depths range from a minimum of 2-3 microns to a maximum of about 40 microns. Elaborate and expensive equipment would be required to generate foam with microscopic bubble sizes considerably smaller than the smallest of such cells, and even with such equipment, it appears extremely doubtful that the Kumins teachings could be followed consistently on a commercial scale. Thus, foamed water based inks have never been employed successfully in commercial rotary gravure paper printing operations. This in spite of the substantial safety and cost advantages that could have been realized by eliminating the industry's continued reliance on solvent based inks.